Project Summary Hyperlipidemia is a common health condition in the transplant patient population, causing end-stage heart disease in approximately 40% of all patients requiring a heart transplant. It develops in 50% of heart transplant recipients after the first year, and 95% of patients within 5 years of transplant. Ischemic heart disease resulting from atherosclerosis and hyperlipidemia is also a major predictor of heart transplant rejection. Hyperlipidemia contributes to acute and chronic rejection in humans, yet, two-thirds of transplants patients remain dyslipidemic despite treatment and a significant number are statin intolerant. Despite its prevalence in the transplant population and its importance in transplant outcome, the role of hyperlipidemia in transplant rejection is poorly understood. We discovered that hyperlipidemia promotes an aggressive rejection response that results in accelerated rejection of allogeneic heart transplants and resistance to tolerance induction by promoting the development of anti-donor CD4+ Th17 cells and altering regulatory T cells. Recently we discovered that hyperlipidemia also leads to significant changes in Virtual Memory (VM) CD8 T cells, a memory T cell population with a memory phenotype (MP) that is in antigen-inexperienced or nave mice. Nave hyperlipidemic mice exhibit a two-fold increase in the number of VM cells in the periphery when compared with controls. VM cells in hyperlipidemic mice also exhibited increased levels of p-STAT3 suggesting that their activation state is significantly altered. To our knowledge, this is the first description of a physiologically relevant concomitant health condition that alters VM T cell subsets. Given the importance of memory T cells in transplant rejection and resistance to tolerance induction, we hypothesize that alteration in VM cells induced by hyperlipidemia can affect transplant outcome and the ability to induce tolerance. The central goal of this proposal is therefore to develop a mechanistic understanding of how hyperlipidemia affects the development of VM cells and determine the role of VM cells in transplant rejection and resistance to tolerance induction. Our specific aims are to (1) Define the mechanisms by which hyperlipidemia results in expansion of VM cells; and (2) Test the hypothesis that VM CD8 cells mediate anti-donor responses that promote rejection and resistance to tolerance induction. The studies described will further our understanding of VM CD8 T cells and conditions that affect them, as well as define their role in transplantation. This work is significant and innovative because it has the potential to define how VM cells are altered in hyperlipidemic individuals and define a role for this population in rejection and resistance to tolerance.